In a blender, combine the vinegar, salt, pepper, and mix until the salt is dissolved. Slowly add the olive oil a few drops at a time. Add the chopped mint. Taste and adjust seasonings,.

Cut watermelon into bite-sized pieces. Remove and discard the seeds, and set aside.

Peel and slice the onion into bite-sized pieces.

In a large bowl, combine the watermelon, onion, olives and feta. Pour the dressing over the melon mixture and toss gently until everything is coated and evenly mixed. Garnish with the remaining mint sprigs.